Electronic controls have been employed for performing various transmission shift control functions, including main or line pressure regulation, shift point scheduling and clutch pressure control during shifting. Such controls are typically much more flexible then the hydraulic controls they replace, enabling the use of mode selection for customizing the transmission operation to suit the operator of the vehicle. Commonly, two or more modes, such as Normal and Performance are designated. In the Normal mode, shifting is scheduled at relatively low engine speed and pressure levels to achieve good fuel economy and a smooth shift feel. In the Performance mode, shifting is scheduled at higher engine speed and pressure levels to achieve improved part-throttle performance and a crisp shift feel.
Shift point scheduling is reasonably straight forward, but clutch pressure control is relatively complicated due to performance changes with wear and age, and part-to-part variability in general. Consequently, some sort of feedback is generally employed to ensure that the desired shift feel/quality is actually achieved.
The present invention pertains to a control in which the supply of fluid to an on-coming clutch during shifting is carried out in accordance with a predetermined pressure schedule, trimmed by an adaptive or learned adjustment. In a specific example of this type of control, described in detail in the U.S. Pat. No. 4,653,350 to Downs et al., issued Mar. 31, 1987, and assigned to General Motors Corporation, a base open-loop pressure schedule provides a desired clutch pressure as a function of time and torque inputs, and the control energizes a solenoid-operated hydraulic valve at a duty cycle or current level for achieving the desired pressure. The duration of the shift is measured and compared to a desired or reference duration, and an adaptive pressure correction term is developed in relation to the difference, if any. In subsequent shifting to the same speed ratio, the pressure command derived from the open-loop pressure schedule is adjusted by the adaptive correction term. In this way, the correction amount provides the feedback for adaptively adjusting the base open-loop control to compensate for the variations referred to above.
The incorporation of both multiple shift pressure moding and adaptive pressure controls poses a significant control difficulty for a number of reasons. For example, adaptive pressure corrections determined in one mode may not apply to the other mode. As a result, the unselected mode does not benefit from the information learned while operating in the selected mode, and in some cases, differentiation between modes may be virtually nonexistent or reversed.